(a) Field of the Invention
This invention relates to methods and compositions for retarding the ambient temperature hardening of a phenolic resole resin when such resin is contacted with a nitroalkane and a substance which hardens the resin under alkaline conditions at ambient temperature. This invention also relates to a hardening composition of a nitroalkane dissolved in an organic ester.
It is desirable to be able to easily control the rate of alkaline phenolic resole resin hardening when ambient temperature hardeners are used such as by being able to retard the gelation and hardening and maintain flowability, trowelability and other working properties of phenolic resole resin compositions. This is particularly the case in warm climates. The phenolic resole resin compositions of this invention include "binder compositions" for binding materials, e.g., an aggregate, or "raw batch compositions" which contain an aggregate and sufficient quantity of the binder composition to bind the aggregate on hardening of the binder.
Applicants have found that the ambient temperature hardening of a phenolic resole resin admixed with a hardening agent such as an organic ester functional hardening agent, lightburned magnesium oxide or mixtures of such esters and lightburned magnesium oxide under alkaline conditions can be retarded by including a nitroalkane in such mixture. The use of the nitroalkane extends the work life of the composition.
Retarding the ambient temperature hardening of a binder composition comprising an alkaline phenolic resole resin, a material which hardens the composition at room or ambient temperature and a retarder such as the nitroalkanes is particularly useful in the foundry and refractory arts. Illustratively, foundry core and molding operations involve mixing binder components with sand using suitable mixing equipment and forming various shapes with the help of pattern equipment. The quality of the cores and molds produced in a foundry determine the quality of castings produced from them. The strengths of cores and molds, as measured by the tensile strength of the mixed sand from which they are prepared, is an important criteria in establishing mold and core quality.
No-bake binder systems, i.e., room or ambient temperature hardening systems, used in foundries depend on their ability to uniformly coat sand grains which then cure into strong, rigid shapes at ambient conditions. One no-bake binder system is the "ester cure" process which uses a highly alkaline phenolic resole resin with a pH above 11 and an alkali to phenol molar ratio of 0.2:1 to 1.2:1. In a typical core and molding operation in a foundry, the binder components are mixed into sand in either a batch or a continuous mode and the mixed sand is discharged into a pattern. The mixed sand begins to cure, also referred to as hardening, immediately and it is essential to compact it in place in order to achieve optimum bonding strength. If the sand "pre-cures" before it is placed and compacted on a pattern, there is a deterioration in the bonding strength achieved. The term "work time" properties of a binder system is an approximation of the useful time available before the sand mix deteriorates due to precuring. A test to determine work time properties actually measures the degree of deterioration of a sand mixed with binder due to precuring. The usefulness of a binder system is greatly enhanced by having good work time properties. This is understandable because many foundry core and mold operations involve other functions like placing risers and sleeves, reinforcing bars, handling hooks, chills, etc., wherein the mixture of sand and resin undergoes varying degree of "pre-cure" before it is compacted in place.
A test to determine work time properties of a binder system involves preparing a sand mix with it. From this sand mix, tensile test specimens are prepared immediately and at periodic intervals. These specimens are allowed to cure and their tensile strengths are determined. The rate at which a binder system looses its strength as a function of pre-cure time, is a measure of its work time properties. The longer work time is shown by higher tensile strengths in the product. Another way to determine or compare relative work time is by determining the length of time it takes to gel a binder system.
The above remarks as to foundry operations also generally apply to refractory operations.
(b) Description of the Prior Art
U.S. Pat. No. 2,529,712 of Nov. 14, 1950 to E. Teague shows the preparation of abrasive products by coating abrasive grains with a phenolic resin and nitroalkane and then mixing powdered shellac with the coated abrasive. At least 50% of the bonding agent is shellac. The abrasive grains which are coated with phenolic resin, a nitroalkane, and shellac are cold pressed into the desired shaped article. The uncured article is then subjected to heat treatment at high temperature such as 160.degree. C. to effect cure. The nitroalkane is described as a plasticizer for shellac in preparing the shaped article.
U.S. Pat. No. 2,531,863 of Nov. 28, 1950 to M. Scott et al. shows the preparation of anion exchange resins by reaction of phenol with formaldehyde in the presence of sodium hydroxide at a temperature of less than 30.degree. C. and the subsequent reaction of such reaction product with a nitroalkane. The remaining nitro group is subsequently reduced to an amino group to prepare the anion exchange resin.
U.S. Pat. No. 2,760,951 of Aug. 28, 1956 to H. Parde et al. shows the reaction of nitromethane, phenol, and formaldehyde in the presence of a condensation catalyst at temperatures above 60.degree. C. to produce thermosetting resins.
U.S. Pat. No. 3,917,558 of Nov. 4, 1975 to J. Gardikes et al. discloses the addition of nitroalkanes to various formaldehyde containing resins such as phenol-formaldehyde which contain free formaldehyde in order to scavenge or reduce the free formaldehyde content of the resin. The scavenger reaction is conducted at a pH above 7, preferably between 8 to 10. The reaction is conducted at temperatures of from about 40.degree. C. to 90.degree. C. for about a half hour to one hour. There is no mention of ambient temperature hardening under alkaline conditions nor the use of ester or lightburned magnesia hardeners in the Gardikes et al. patent.
U.S. Pat. No. 4,341,668 of Jul. 27, 1982 to J. Martin et al. describes trimethylol nitromethane as a compound suitable for providing methylene groups at a temperature above 135.degree. C. for post-cure of cellulose which is impregnated or bonded with a composition containing an isocyanate in a hydrophobic diluent, an aqueous phenol-formaldehyde resin, and the trimethylol nitromethane.
U.S. Pat. No. 4,379,866 of Apr. 12, 1983 to F. Henry, Jr. et al. describes the use of tris(hydroxymethyl)nitromethane, also referred to as trimethylol nitromethane, as the formaldehyde-source, cross-linking agent for a phenol-formaldehyde or resorcinol polymer resin binder system for a plugging refractory's ability to withstand residence periods in a mud gun without premature solidification.
Canadian Patent 2,045,201 of Dec. 23, 1991 to J. Dieter et al describes the reduction of the formaldehyde content of a phenolformaldehyde based paint and solvent mixture by adding a nitroalkane to the mixture.
Technical Bulletin "TB 60" of the Angus Chemical Company discusses NIPAR 640 which is a mixture of nitroparaffins such as nitroethane and nitropropanes. The second page of that bulletin shows a composition of 20% NIPAR 640 and 80%, by weight, of isopropyl acetate as a thinner for acrylic ink. However, the isopropyl acetate is not an ambient temperature hardening agent for applicants, compositions having a high pH and high concentration of alkali metal as used in the ester cure process. It was found that the isopropyl acetate was immiscible in the highly alkaline resin.